Power Amp Tube First Aid - Rivera Knucklehead Reverb KR 55T Owner's Manual

Turn off the amp, and allow the tubes to cool. Now pull out the troublesome tube and
replace it with one of the same value (that is, if you're pulling out a 12AX7, replace it
with a 12AX7).
Make sure that the tube is oriented correctly when pulling it out or putting it back in. If
you look at the end of the tube and the socket, you'll notice that the nine pins are
arranged in an incomplete circle. Always make sure the pins are aligned correctly.
Never force a tube into its socket.
Remember to put the sleeve back on after you check or replace a tube.

Power amp tube first aid

Like Preamp tubes, power amp tubes can go bad or wear out. Your Knucklehead
Reverb has two or four power amp tubes, and if one goes bad, they should all be
replaced. This assures optimum output and tone.
If a power tube shorts out, the HT Fuse will be blown. Remove power from the amp and
replace the fuse before doing the following:
Remove the rear Grille. Let the power tubes cool. Remember the way the eight pins
are arranged, and note that the center hole on the tube socket has a keyway that
matches the center post on the tube.
Replace one tube. Turn the amp on. If the fuse blows (or the tube glows cherry red,
indicating an internal short), you've found the bad tube. Turn off the amp immediately.
If the fuse doesn't blow, replace another tube and turn the amp on again. Repeat this
procedure until you've determined which tube is bad.
Knucklehead Reverb 1.0 p 35